1. Field of the Invention
The present invention relates to antennas for portable telephones and, in particular, to a portable telephone antenna circuit having reduced susceptibility to the human body and a method for realizing the same.
2. Description of the Related Art
A portable telephone, such as a cellular telephone or a cordless telephone, generally employs a helical antenna, a .lambda./4 antenna, a .lambda./2 dipole antenna or a .lambda./2 monopole antenna. However, the helical antenna is rarely employed in a portable telephone, since it has poor antenna characteristics. For example, a significant amount of power is converted to heat due to the high inductance of the helical antenna.
Moreover, the .lambda./4 antenna, the .lambda./2 dipole antenna and the .lambda./2 monopole antenna, which are commonly employed in a portable telephone, are susceptible to current leakage through the human body, as shown in FIGS. 1A to 1C. FIGS. 1A to 1C are diagrams illustrating current leakage caused when the various antennas applicable to a portable telephone come into contact with the human body. The leakage current flows through the user's body, when the user holds the portable telephone to his/her ear to converse using the telephone. An increase in the leakage current brings about a reduction in the coverage area of the portable telephone. Referring to FIGS. 1A to 1C, the .lambda./4 antenna (see FIG. 1A) has the largest current leakage, the .lambda./2 dipole antenna (see FIG. 1C) has the second largest current leakage, and the .lambda./2 monopole antenna (see FIG. 1B) has the least largest current leakage. That is, among the antennas of FIGS. 1A to 1C, the .lambda./2 monopole antenna can offer the widest coverage area to the portable telephone. For this reason, the .lambda./2 monopole antenna is widely employed in portable telephones.
Nevertheless, when the portable telephone employing the .lambda./2 monopole antenna comes into close contact with the human body, the standing wave ratio (SWR) increases, thus deteriorating the antenna characteristics. The increase in the SWR results in a reduction in the coverage area of the portable telephone.
Of course, the antenna for a portable telephone can be designed in fill consideration of such antenna characteristics. In such a case, however, the antenna is relatively longer in length than the body of the portable telephone. For example, in the case when the .lambda./2 monopole antenna is employed in a 900 Mhz cordless telephone, the antenna should be about 16 cm, which is much longer than the body of an ordinary cordless telephone.